1. Field of the invention:
This invention relates to an image processing apparatus such as a copying apparatus and image reading apparatus. More particularly, it relates to a copying apparatus comprising a density detection device for detecting the density of an original to be copied, in a process of prescanning the original, and an adjusting means for adjusting at least one of the conditions of copying the original, on the basis of a group of detected density data.
2. Description of the prior art:
In a copying apparatus, the copy density (contrast) is adjusted by controlling one or more copy parameters such as the voltage level applied to the exposure lamp and the potential of the photosensitive body, in accordance with the density of an original to be copied. Usually, the density of an original is detected by prescanning the original to automatically adjust the density of a copy of the original. More specifically, an original is scanned by the light of an exposure lamp before the actual copy process, so that the light reflected from the original impinges on a photosensor. The data corresponding to the density of various portions of the original (density data) are obtained from the sensor. One or more of the above-mentioned copy parameters are automatically adjusted on the basis of the accumulated value, a mean value, maximum value or minimum value of these density data, thereby enabling improvement of the image quality of a copy which will be obtained in the subsequent copying process. Then, the actual copying process is performed under the thus adjusted parameters.
In a practical use, however, a copying apparatus must copy a very wide variety of originals, and therefore each copying apparatus must cope with several kinds of originals. Particularly, it is more usual for a color copying apparatus to deal with an original having black areas (such as photographs) than an original having lines (such as characters) only. When copying an original in the form of a thick book, moreover, it is often that the copy process is conducted without covering the original (i.e., without using an original cover), to facilitate the handling the original, which results in portions outside the original being detected as black areas. In these cases, the density data obtained in a prescanning process includes those obtained from black areas or blank areas caused by the nonuse of an original cover, resulting in the density data failing to correspond to the actual density of the original. Namely, when an arithmetic mean value of the density data is used (mean value method), the mean value is biased toward a black tone so that the following copying process cannot be conducted properly (the density of the resulting copy is too low).
Moreover, a copying apparatus using the mean value method often fails to detect properly the density of an original. For example, when an original having a wide white background is to be copied, the mean value of the density data represents a smaller value than that which is necessary for a proper copy of the original, resulting in obtaining a copy in which the image area, which is most important, becomes too thick in density. In order to overcome this disadvantage, an improved copying apparatus has been developed. In an improved apparatus, the density data obtained from areas of an original, the density of which is greater than an upper threshold level or less than a lower threshold level, are eliminated from the density data to be further processed, so that the copying parameters are adjusted on the basis of only the density data obtained from the intermediate density areas of the original.
In such an improved copying apparatus, however, those upper and lower threshold levels are preset at fixed values during manufacture of the apparatus in a factory. Unevenness in the light strength of an exposure lamp or in the sensitivity of a photosensor and the deterioration of the sensitivity may cause the data obtained from an intermediate density area to be deviated toward either of the darker and brighter densities. This deviation of the intermediate density area makes the density detection performed in a prescanning process entirely useless, especially when an original having pale or thick density images is to be copied. In this way, a prior art copying apparatus having the density detection device has the problem that it cannot detect accurately the density of an original, depending on the variation of the characteristics in each apparatus or on the kind of the original. Even if it is designed so as to allow the correction of the threshold values, the prior art still involves the problem that it is necessary to verify the correction by performing a test copy, causing a prolonged time for the correction and wasted copy paper.
Further, such a conventional copying apparatus has a further problem as described below. A photosensor such as a photo transistor has a light receiving face F of a few millimeters in diameter (FIG. 11(B)). When an original B is to be copied wherein, as shown in FIG. 11(C), the image density suddenly changes from black to white (i.e., there is no intermediate density area between the black area B1 and the white area B2), the output of the sensor moving in the direction of the arrow (FIG. 11(B)) varies as indicated in FIG. 11(A) from a value corresponding to black to another value corresponding to white, following a curve containing intermediate values corresponding to half tones. This is because the light receiving face F extends over both the black area B1 and the white area B2 during when the light receiving face F moves from a first position T1 to a second position T2. Namely, the output of the sensor is affected by both the black and white areas B1 and B2 during when the light receiving face F moves between the positions T1 and T2. This is also applicable in the case that either of the black and white areas is replaced with a photograph having an intermediate density. In this case, the photograph borders the black area or white area so that the density data obtained immediately before and after the border contains large errors.
The above will be described more specifically. As shown in FIG. 12(A), an original C having photographs 51 and 52 and white areas 53 to 55 surrounding the photographs is placed on an original table 2. One example of the actual image density data of such an original is shown in FIG. 12(B). When the original is prescanned, the density data obtained as outputs of the sensor will be represented by a distorted waveform as shown in FIG. 12(C). In FIGS. 12(B) and 12(C), the one dot chain lines represent the upper and lower threshold levels of the density data, respectively. When only the density data existing between the upper and lower threshold levels are considered (i.e., density data corresponding to black and white are eliminated from the density data of (FIG. 12(C)), it will be easily seen that the density data obtained immediately before and after the borders have extremely appreciable errors. Hence, a prior art copying apparatus has a drawback in that density data containing errors are utilized to cause an imprecise control of the copying parameters.
The above-mentioned difficulties of a prior art copying apparatus are applicable also to another image processing apparatus, that of an image reading apparatus such as an image scanner.